Gradient tungsten-doped Bi3TiNbO9 ferroelectric photocatalysts with additional built-in electric field for efficient overall water splitting

Bi3TiNbO9, a layered ferroelectric photocatalyst, exhibits great potential for overall water splitting through efficient intralayer separation of photogenerated carriers motivated by a depolarization field along the in-plane a-axis. However, the poor interlayer transport of carriers along the out-of-plane c-axis, caused by the significant potential barrier between layers, leads to a high probability of carrier recombination and consequently results in low photocatalytic activity. Here, we have developed an efficient photocatalyst consisting of Bi3TiNbO9 nanosheets with a gradient tungsten (W) doping along the c-axis. This results in the generation of an additional electric field along the c-axis and simultaneously enhances the magnitude of depolarization field within the layers along the a-axis due to strengthened structural distortion. The combination of the built-in field along the c-axis and polarization along the a-axis can effectively facilitate the anisotropic migration of photogenerated electrons and holes to the basal {001} surface and lateral {110} surface of the nanosheets, respectively, enabling desirable spatial separation of carriers. Hence, the W-doped Bi3TiNbO9 ferroelectric photocatalyst with Rh/Cr2O3 cocatalyst achieves an efficient and durable overall water splitting feature, thereby providing an effective pathway for designing excellent layered ferroelectric photocatalysts.

with present results.The novelty of this work may be not high enough to be considered for publication on this journal.Other comments are listed below, 1.The authors should highlight the innovation in introduction.2. Whether the XRD peaks are shifted after W doping. 3. Add the scale in Fig. 2e and Supplementary Fig. 7 4. Please provide specific XPS etching conditions, and compare sample thickness with etching depth. 5.The author mentions that "the W/(Ti+Nb) ratio on the surface is much higher than that in the bulk for all W-doped samples".How does this affect the catalytic activity.6.It is important to determine the stability of Ru single atomic sites during photocatalytic overall water splitting.It would be valuable to present high-resolution TEM images of W-doped Bi3TiNbO9 after photocatalytic water splitting to assess any changes in their structure.7. The authors used the molten salt method to prepare the catalyst, what was the ratio of NaCl and KCl, what was the melting point, and what was the basis of the choice 8. What form is W doped in?Single atom?Could you elaborate further?9.More specific experimental conditions need to be given, including light intensity, spectral range, reaction temperature, distance between lamp and reactor, etc. 10.The authors have done too little work on the water splitting mechanism, and should analyze the activity enhancement in depth with theoretical calculations 11.The authors should give a table to compare the performance of this work with other recently published works.12.The data of the energy band structure of the materials should be supplemented to make the paper more convincing.13.The manuscript will benefit from further careful proofreading to correct for errors in grammar, syntax and the selection of appropriate phrases to convey the intended message.
Reviewer #3 (Remarks to the Author): Recommendation: Minor revisions Comments: This manuscript demonstrates a gradient doping strategy that introduces an additional electric field in the c-direction of the Bi3TiNbO9 nanosheets while simultaneously enhancing the polarized electric field in the a-direction.This strategy enhances the spatial separation efficiency of photogenerated carriers and achieves an efficient and durable overall water splitting.This manuscript provides a useful reference for the design of high-performance layered ferroelectric photocatalytic materials.I propose to publish it with minor revisions.The specific revisions are as follows: 1.In lines 75-76 of the manuscript, how to understand "modification" in the modified flux method?From the specific preparation process described by the authors, it is a conventional flux method.2. Characterization of the ferroelectric polarization intensity (Ps) of single nanosheet is not easy, and what is the intrinsic physical mechanism of the piezoelectric coefficient d33 characterizing the Ps of Bi3TiNbO9 nanosheets (Lines 171-173 of the manuscript)?3.In lines 233-235 of the manuscript, the authors did not consider the effect of the specific surface area of the catalyst when comparing the hydrogen and oxygen evolution activities before and after doping.The effect of specific surface area can be excluded after using specific surface area normalization, and more accurate results can be obtained.4. In lines 235-238 of the manuscript, the authors explain that once the doping exceeds the limit, the impurity phase formed may be enriched on the surface of the sample, thus affecting the light absorption properties and reducing the photocatalytic activity.However, from XRD and SEM, no impurity phase was observed even with doping up to 12%.In addition, the results of UV-visible absorption spectra showed that the higher the doping amount, the higher the light absorption (redshift).Therefore, are there some other reasons regarding the reduced catalytic activity due to overdoping?

Reply to Reviewers' Comments
Reviewer #1: The search for efficient and stable photocatalysts for overall water splitting has long been targeted and attracted worldwide attention in the past decades.Any important progress in this topic is highly expected and greatly welcome in solar photocatalysis community.This impressive study by Huang et al. reported a novel gradient tungsten-doped Bi3TiNbO9 ferroelectric photocatalyst that possesses not only the strengthened intrinsic depolarization field magnitude within the layers along a-axis but also an additional electric field along c-axis.This enables the desirable anisotropic migration of photogenerated electrons and holes towards the basal {001} surface and lateral {110} surface of the Bi3TiNbO9 nanosheets, respectively.As a result, the gradient doped ferroelectric photocatalyst with Rh/Cr2O3 cocatalyst shows an efficient and stable photocatalytic overall water splitting.Overall, the results achieved in this study are significant and could provide important implications for developing a new class of lead-free ferroelectric photocatalyst for overall water splitting.Moreover, the authors provide solid evidence for the clear relationship between structure (motivation) and activity (target) of photocatalysts by conducting comprehensive characterizations.Considering the high quality and potential great impact of this study, I am very glad to recommend its publication in Nature Communications.On the other hand, I also believe that the proper addressing of the following comments should be beneficial for this study.

Response:
We thank the reviewer for the positive comments and constructive suggestions.Please find our point-to-point replies to your comments.
1.For the substitutional doping of cations in metal oxides for metal dopants, the ionic radius and oxidation state of metal dopant are two most important parameters to be considered.The information on this concern is absent and needs to be added.It is also useful to discuss or predict the effect of other dopants in the same group of W dopant on the photocatalytic activity.

Response:
We thank the reviewer for this suggestion.The ionic radius and oxidation state of the dopant are important parameters affecting the doping sites.Following your suggestion, we have compared the relevant information in Supplementary Table 1.However, it is difficult to predict the effect of doped ions on photocatalytic activity, considering that the different local energy levels of dopants.Our XPS results and added theoretical results consistently suggest that W dopant tends to occupy Nb sites.

Response:
The UV-visible absorption spectra (Supplementary Fig. 10) show that the absorption band edge position of Bi3TiNbO9 is 400.8 nm and the absorption band edge position of W doped Bi3TiNbO9 is about 414.8 nm.We tested the photocatalytic overall water splitting activity of W doped Bi3TiNbO9 under visible light irradiation (λ > 400 nm).However, no obvious activity was detected largely due to the weak visible light absorption of W doped Bi3TiNbO9.
3. It is useful to compare the electronic structure of Bi3TiNbO9 before and after W doping.The authors are encouraged to conduct theoretical calculations and analyze the results.By doing this, the possible changes of both bandgap and charge mobility can be obtained.
Response: Following this suggestion, we have added the DFT calculation of the electronic structure of Bi3TiNbO9 and Bi3TiNbO9-W in revised Fig. 3.The results show that the bandgap values of W-doped Bi3TiNbO9 decrease with the increase of W-doping concentration, in which the valance band maximum (VBM) keeps nearly unchanged, and the conduction band minimum (CBM) is shifted downward.Consequently, a built-in electric field can be formed for Bi3TiNbO9 with gradient W doping.According to the spatial distribution of W-dopants achieved in this work, the built-in electric field induced by gradient W-doping is illustrated in revised Fig. 3d.
Obviously, such an additional built-in electric field could provide a driving force for the migration of photoexcited electrons from bulk to the surface of W-doped Bi3TiNbO9.4. Based on the detailed study of dopant concentration dependent photocatalytic overall water splitting activity (Fig. 5a), the optimal doping concentration of W to Ti and Nb was 5 at%.It is Fig. 7.This correlation should be further analyzed and discussed.This finding is very important for designing highly efficient doped photocatalysts.

Response:
The improvement of photocatalytic overall water splitting activity of Bi3TiNbO9 caused by W 6+ ion doping is mainly due to the enhanced built-in electric field promoting the transfer and separation of photogenerated carriers.Therefore, within the appropriate doping concentration range, the built-in electric field strength of the material is positively correlated with the doping concentration so that the photocatalytic activity is also continuously improved.Due to the limitation of structural tolerance, excessive dopants are difficult to be incorporated into the lattices and thus tend to be enriched on the surface of photocatalytic materials, limiting the photocatalytic activity.We also made relevant analysis in the manuscript."As shown in Fig. 6a, the photocatalytic performance displays a volcanic curve with the W/(Ti+Nb) ratio increasing, which is consistent with the trend of structural distortion (inferred in Supplementary Fig. 2b, 3).
When the ratio of W/(Ti+Nb) reaches 5%, that is, the saturation concentration of doping (Supplementary Fig. 7), Bi3TiNbO9-W exhibits the optimal photocatalytic overall water splitting activity with the average hydrogen and oxygen evolution rates of 43.99 and 20.66 μmol h -1 , respectively, which are 5.55 times higher than those of Bi3TiNbO9.Once the doping amount exceeds the limit value, the formed impurity phase may be enriched on the surface of the sample, affecting light absorption characteristics and reducing photocatalytic activity." 5. It is remarkable that the surface photovoltage signal was reversed from positive to negative by the gradient W doping in Figure 4.This could be the strong evidence for the formation of an additional built-in electric field, which is the most important feature of the developed ferroelectric photocatalyst.The authors should highlight the correlation between the reversed photovoltage signal and formation of the additional built-in electric field.
Response: Following this suggestion, we have added the following description in revised Supplementary Fig. 17. "Both Bi3TiNbO9 and Bi3TiNbO9-W exhibit n type semiconductor characteristics with upward surface band bending.Since the interlayer barrier and the surface space charge layer restrict the migration of photogenerated electrons to the surface (equivalent to the inherent built-in electric field, Ei, pointing from the bulk phase to the surface), Bi3TiNbO9 presents a positive SPV signal.When the direction of the additional built-in electric field (E) induced by gradient doping is opposite to Ei, the positive SPV signal intensity will be weakened, and even the direction of the SPV response will be reversed, for example, the W gradient doping in Bi3TiNbO9 presents a negative SPV signal.However, when the direction of the additional built-in electric field induced by gradient doping is the same as Ei, the positive SPV signal intensity will be enhanced".

Reviewer #2:
This manuscript prepared W doped Bi3TiNbO9 nanosheets by a modified flux method for photocatalytic overall water splitting.The preparation method and some characterizations are presented.Notably, a similar work has just been reported by the authors in Advanced Science (Selective Exposure of Robust Perovskite Layer of Aurivillius-Type Compounds for Stable Photocatalytic Overall Water Splitting, Adv.Sci. 2023, 2302206).In this work, the authors tried to dope a W-layer into the interlayer, which was not solidly supported the successful addition of this layer with present results.The novelty of this work may be not high enough to be considered for publication on this journal.
Response: We greatly appreciate this reviewer for the great contribution in reviewing our manuscript, especially for the constructive comments and suggestions.Please find our point-topoint replies to your comments.
1.The authors should highlight the innovation in introduction.
Response: Following this constructive suggestion, we have highlighted the innovation of this study in the introduction of the revised manuscript as follows."Although reducing the thickness of the layered material along the c-axis to obtain an ultrathin structure or selective exposure of different layers can shorten the migration distance of photogenerated electrons from the bulk to surface, the nature of poor interlayer charge transport remains unchanged.Therefore, the problem of large interlayer charge transport barrier remains a bottleneck in designing efficient layered Bi3TiNbO9 catalysts for overall water splitting.Here, we introduce an additional built-in electric field, perpendicular to the depolarization field in Bi3TiNbO9 nanosheets, by tungsten (W) element doping induced energy band structure adjustment between surface and bulk to break such a bottleneck.Typically, donor dopants can effectively increase the number of free electrons, thus raising the corresponding Fermi level of semiconductors." More importantly, the gradient W doping realized in this study causes two remarkable features that have never been realized in Bi3TiNbO9.One is the generation of an additional electric field along the c-axis and the other is the simultaneously enhanced magnitude of depolarization field within the layers along the a-axis due to strengthened structural distortion.
2. Whether the XRD peaks are shifted after W doping.

2.
Substitutional tungsten doping in wide-bandgap metal oxide semiconductors usually causes some additional visible light absorption due to its low electron levels.According to UV-visible absorption spectra in Supplementary Fig.10, the W doped Bi3TiNbO9 also has additional marginal visible light absorption.Fig.5shows the 5.5 times enhancement in photocatalytic overall water splitting under UV-visible light irradiation.How about the photocatalytic overall water splitting activity only under visible light irradiation?Please provide this information.

Fig. 3
Fig. 3 Additional built-in electric field introduced by gradient doping.a, The calculated band structures of Bi3TiNbO9 with different W-doping concentrations.The calculated bandgap values are also given.The VBM (set to be 0 eV) and the Fermi level are denoted by black dotted and red dashed lines, respectively.The insets show an enlarged distribution of the Nb-4s energy levels highlighted by the cyan dashed rectangles.The black dashed lines in the insets represent the averaged energy of the Nb-4s levels.b, The calculated density of state (DOS) of Bi3TiNbO9 with different W-doping concentrations.The DOS within an energy window from 2.0 to 3.5 eV nearby the CBM is shown in band structure plots.For the cases of B-doped Bi3TiNbO9 with doping concentration of 3.2, 6.3, and 12.5%, the local DOS value of W dopant is multiplied by a factor of 16, 8, and 4, respectively.c, The band alignment of pristine Bi3TiNbO9 and W-doped Bi3TiNbO9 with different doping concentrations, by taking the Nb-4s energy level as a reference.d, Schematic diagram for the built-in electric field of Bi3TiNbO9 induced by gradient W-doping.

Table 1
The cation radius of the perovskite layer in Bi3TiNbO9 and the VIB group in the periodic table of elements Hosted by the Atomistic Simulation Group in the Materials Department of Imperial College --